java基础：9.1 抽象类

在前几章，我们学了父类与继承等操作。有时候，一个父类设计得非常抽象，以至于它都没有任何具体的实例。这样的类称为抽象类。

在父类里定义一些方法，但这些方法不能在父类中实现，因为他们的实现取决于几何对象的具体类型。这样的方法称为抽象方法，在方法头中用abstract修饰符表示。抽象方法是非静态的

抽象类和常规类很像，但是不能使用new操作符创建它的实例。抽象方法只有定义而没有实现，它的实现由子类提供。

一个包含抽象方法的类必须声明为抽象类。

抽象类的构造方法定义为protected，因为它只被子类使用。创建一个具体子类的实例时，它的父类的构造方法被调用以初始化父类中定义的数据域。

不能使用new 操作符从一个抽象类创建一个实例，但是抽象类可以用作一种数据类型

GeometricObject[] object = new GeometricObject[10];

object[0] = new Circle();  是合法的

// GeometricObject.java
// 包含抽象类的父类
public abstract class GeometricObject {

	private String color = "white";
	private boolean filled;
	private java.util.Date dateCreated;
	
	protected GeometricObject() {
		dateCreated = new java.util.Date();
	}
	
	protected GeometricObject(String color,boolean filled) {
		dateCreated = new java.util.Date();
		this.color = color;
		this.filled = filled;
	}
	
	public String getColor() {
		return color;
	}
	
	public void setColor(String color) {
		this.color = color;
	}
	
	public boolean ifFilled() {
		return filled;
	}
	
	public void setFilled(boolean filled) {
		this.filled = filled;
	}
	
	public java.util.Date getDateCreated(){
		return dateCreated;
	}
	
	@Override
	public String toString() {
		return "created on "+dateCreated + "\ncolor" + color + 
				"and filled :" + filled;
	}	
	public abstract double getArea();
	public abstract double getPerimeter();
}

// Circle.java
// 子类1
public class Circle extends GeometricObject{
	   private double radius;
	   public Circle(){
	    }
	   
	   public  Circle(double newRadius){
	        this.radius = newRadius;
	    }
		
	   protected  Circle(double radius,String color,boolean filled) {
			this.radius = radius;
			setColor(color);
			setFilled(filled);
		}
	    /* return radius */
	    public double getRadius() {
	    	return radius;
	    }
	    /* set radius */
	    public void setRadius(double radius) {
	    	this.radius = (radius >=0) ? radius : 0;
	    }
	    
	    public double getArea(){
	        return radius*radius*Math.PI;
	    }
	    
	    public double getPerimeter(){
	    	return 2*radius*Math.PI;
	    }
	    
	    public void printCircle() {
	    	System.out.println("the circle is created " + getDateCreated() +
	    			"and the radius is " + radius );
	    }
}

// Rectangle.java
// 子类2
public class Rectangle extends GeometricObject{
	   private double width;
	   private double height;
	   
	   public Rectangle(){
	    }
	   
	   public  Rectangle( double width,double height){
	        this.width = width;
	        this.height = height;
	    }
		
	   protected  Rectangle(double width,double height,String color,boolean filled) {
	        this.width = width;
	        this.height = height;
			setColor(color);
			setFilled(filled);
		}

	    public double getWidth() {
	    	return width;
	    }
	    
	    public void setWidth(double width) {
	    	this.width = width;
	    }

	    public double getHeight() {
	    	return height;
	    }

	    public void setHeight(double height) {
	    	this.height = height;
	    }
	    
	    public double getArea(){
	        return height*width;
	    }
	    
	    public double getPerimeter(){
	    	return 2 * (height+ width);
	    }    	    
}

// 测试程序

public class TestGeometric {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		GeometricObject g1 = new Circle(5);
		GeometricObject g2 = new Rectangle(5,3);
		System.out.println("The two objects have the same area? "
				+ equalArea(g1,g2) );
		displayGeometricObject(g1);
		displayGeometricObject(g2);
	}
	
	public static boolean equalArea(GeometricObject g1,GeometricObject g2) {
		return g1.getArea() == g2.getArea();
	}
	
	public static void displayGeometricObject(GeometricObject g) {
		System.out.println();
		System.out.println("the area is " + g.getArea());
		System.out.println("the perimeter is " + g.getPerimeter());
	}

}